In an image reading apparatus, a document is irradiated with light and the light reflected by the document is received, whereby the image printed in the document is converted into electrical signals. In such an image reading apparatus, a linear light source device capable of emitting linearly extended light is often used as the light source to irradiate the document.
FIGS. 24 and 25 show an example of conventional linear light source device. The linear light source device X shown in the figures includes two light sources 91 and a light guide member 92. Each of the light sources 91 includes three LEDs 91a for blue light, green light and red light. The light guide member 92 is made of a light transmitting material such as acrylic resin. The light guide member 92 is elongated in the right and left direction in FIG. 24. The light guide member 92 includes two light incident surfaces 92a, a reflecting portion 92b and a light emitting surface 92c. The two light incident surfaces 92a are positioned at longitudinally opposite ends of the light guide member 92. The light sources 91 are disposed in front of the respective light incident surfaces 92a. The reflecting portion 92b is provided on the lower surface of the light guide member 92. The light emitting surface 92c is provided on the upper surface of the light guide member 92. The light emitting surface 92c is elongated in the longitudinal direction of the light guide member 92 and is arcuate in cross section.
The light emitted from the light sources 91 enters the light guide member 92 through the light incident surfaces 92a. The light traveling in the longitudinal direction of the light guide member is reflected upward by the reflecting portion 92b. The light emitting surface 92c increases the directivity of the light reflected by the reflecting portion 92b to form linear light. The linear light is emitted upward to irradiate the document which is the target of image reading by the image reading apparatus.
In the linear light source device X, however, the brightness of the emitted light may not be uniform in the longitudinal direction. Specifically, the brightness distribution in the longitudinal direction depends on the amount of light reflected by the reflecting portion 92 in the process of traveling within the light guide member 92 in the longitudinal direction after being emitted from the light source 91. For instance, light with a relatively high brightness is likely to be emitted from a portion of the light emitting surface 92c which is close to the light source 91. Thus, the document is not irradiated uniformly, and hence, may not be read properly by the image reading apparatus.    Patent Document 1: JP-A-H9-200439